Gaseous discharge device



K. H. KINGDON ETAL GASEOUS DI S CHARGE DEVI CE Filed oct. 1s, 1941 .5 la "il Sel/ /eaves @un l ll KA A 77h76 Inventors:

Kenneth H Kngclon,

Elliott J. Lawton Attorney.

Patented Mar. 4, 1947 GAsEoUs DISCHARGE DEVICE Kenneth H. eingabe and Elliott- .1.1Lawton,

Schenectady, N. Y., assignors tcGeneral Electric Company, a corporationof New York Application october 13, 1941,' serial No'. 414,710'

4 Claims. (Cl. Z50-27.5)

The present invention relates to improvements in gaseous discharge devices of the controlled discharge type.

Y Gas filled discharge tubes have long been used as a switching means for controlling the flow of relatively large quantities of power, the control function being accomplished by the use of a grid electrode interposed between the anode and cathode of the tube. In the typical mode of use of such a tube the tube is maintained wholly nonconductive by the grid until the grid voltage is raised above a critical level, whereupon an arclike discharge, involving ionization of the gaseous content of the tube, is abruptly initiated.

In certain types of circuits, especially in timing circuits for electrically controlling the detonation of explosive projectiles and the like, it is desirable to employ control means having the general characteristics of gaseous discharge tubes but possessing a higher degree of firing accuracy and reproducibility than is commonly provided by such tubes. It is a primary object of the present invention to make available an improved form of discharge tube which possesses the required ring accuracy and uniformity and which is otherwise suited for the particular application specified' above.

It is found that an important cause of inconsistent operation of controlled gas-filled tubes lies in the characteristics of the cathodes ordinarily employed in these tubes. For example, cathodes which consist of a heated metal body coated with an Vemission venhancing substance such as an alkaline earth oxide tend to be erratic in their emissive qualities and, when used in connectionv with a control grid, lead to relatively unpredictable iiring performance. Uncoated metal bodies, such as tungsten wires, are much more consistent in their properties but, because of their relatively limited emission, are not inherently well adaptedr to sustain an arc-like discharge of the type which characterizes the gas-lled tube. i

According to the present invention, highly consistent operation is combined with satisfactory emission by the use of a composite cathode structure involving both an uncoated metal surface which serves as a primary emitter and a specially conditioned surface which is capable of sustaining an intense discharge after such a discharge is body having a coating of an insulating substance which is adapted to promote eld emission from. the body in the presence of ionization. With combination, employed in conjunction with a control grid, initial current flow: occurs from the heated filament whenever the control grid voltage attains an appropriate level. Thereafter, positive ions produced inthe discharge space by electrons projected from the filament collect on the insulation-covered surfaces of the auxiliary cathodeA part and, assuming these surfaces to be of proper character, produce localized gradients of sufficient intensity to initiate cold-cathode emission from the adjacent metal parts of the cathode. In this way an arc is struck to the parts in question, and full breakdown'of the tube ensues. The transfer from the condition of limited current flow permitted by the filamentary portion of the cathode to a high intensity'arc-discharge supported by the auxiliary cathode part occurs substantially instantaneously, sothat the instant of ring of the tube is dependent almost entirely upon the characteristics of the tungsten filament. Since these are determinable and reproducible with a high degree of accuracy, tubes having the specified construction are uniform in their operation within a very small margin of error.

v The features which :we desire -to protect herein are pointed out with particularity in the appended claims. The invention itself, together with further objects and advantages thereof may best be understood by reference 'to the following description taken'in connection with the accompanying drawing in whichv Fig. 1 shows in section a discharge device suitable embodying the invention, the device Abeing illustrated in connection with a mounting arrangement; Fig. 2 is an enlarged view of the cathode ofthe device of Fig. 1; Fig. 3 is a fragmentary enlargement of a part oi the structure .of Fig.` 2; Fig. 4 is a still further enlarged 'representation'of one of the elements of Fig.3; Fig. 5 illustrates diagrammatically a typical mode of use of the invention; and Fig. 6 is a graphical representation useful in explaining Fig.` 5.

Referring particularly to Fig. 1, there is shown a gaseous discharge tube I0 which is indicated as being mounted axially within a hollow metal shell II; for example, the casing of an explosive projectile such'asvan anti-aircraft shell. The tube IS comprises a pair of glass cylinders I3 and I4 which are separated by a metal ring I5 sealed between them land which are closed at their respective extremities by means of transverse metal headers i1 and I8.` The glass and metal parts referred to are. sealed together in vacuum-tight relation and to .this vend should be constituted of materialsiwhich are capable of being hermetically joined. Thus, the parts I3 and I4 may consist of a borosilicate glass, and the parts I5, I1 and I8 of a metal (e. g. a nickel-iron-cobalt alloy) capable of sealing to glass of this character.

The upper metal header I'I, which forms the anode electrode of the device, is provided centrally with a sealed-off metal tubulation adapted to be used during the fabrication of the tube for evacuating it and for introducing a suitable gaseous iilling. (For the latter purpose one may use a gas such as argon or neon at a pressure of from a few microns to a few millimeters of mercury, and in a particular case it hasV been found advantageous to use argon at a pressure of about 400 microns.) A terminal 2i connected to the tubulation 20 provides a means for connecting the anode part I1 to a suitable potential source (not shown in Fig. 1).

The intermediate ring I5 is provided with a transversely extending mesh 23 which is adapted to serve as a control grid for the tube. Control potential may be applied to the mesh through a terminal conductor 25 which is connected externally to the ring I5.

The header I3, in addition to serving as a closure member and base for the tube, further provides a support for a composite cathode structure having a lamentary part 21 and an auxiliary part 28 which will be described more fully at a later point. The filament 21 is formed of uncoated metal and preferably of tungsten, the latter material being considered most suitable because of its ability to be maintained at a temperature of eiiective thermionic emission without excessive vaporization. The filament may advantageously be on the order of 0.6 mil in diameter in order to permit it to be heated to an emitting temperature from a supply source of limited current capacity. The filament is mounted between a bracket 33, which is in the form of a rectilinearly bent piece of metal secured tothe header I8, and a .relatively rigid support rod 30 which is insu latingly sealed through the header by means of a glass-to-metal seal indicated at 3I. In order to facilitate attachment of the l'ament to the supporting members the ends of the filament wire may be preliminarily pressed between pieces of nickel foil and these pieces welded to the mounting elements as indicated at and' 36.

' The auxiliary cathode portion 28 is shown in enlarged detail in Figs. 2 to i inclusive. In the. illustrated embodiment it comprises a folded piece of nickel 39 (Fig. 3,) which embraces a tab of aluminum foil 4I between its opposed surfaces and which is welded to the upper end of the mounting bracket 33. The aluminum foil 4I, which may be of the order of 2 mils in thickness, is preferably of the type known as anodized aluminum. This means that it is provided on its surface with a thin insulating lrn constituted of aluminum oxide or of a more complex aluminum compound, depending upon the circumstances of preparation of the foil. OneV method of preparing or ,forming anodized aluminum comprises making the aluminum the positive electrode of an electrolytic- 4I (e. g. of aluminum oxide or the like, as above specified) which has a thickness of a fraction of a mil and which possesses minute perforations or crevices as indicated at a, these being very much magniiied in the drawing for the sake of clarity. In order to assure contact between the aluminum foil and the nickel clamping means 39 in spite of the presence of the film 4I the foil may be pricked or perforated at numerous points as indicated at 42 so that the edges of the perforations provide protruding contact points..

In the use of a cathode of the character described, heating current is passed through the filament 21 by the application of potential between the support rod 3u and the header I 8. As a consequence the filament is maintained in emissive condition, although the resulting electron supply is relatively limited because of the small size of the wire. As long as the potential of the grid 23 is maintained below a critical level, no current.

is permitted to ow to the anode Il. However, as soon as the grid potential exceeds the critical value, current iiow begins, and ionization of the gas lling of the tube ensues. Some of the positive ions thus produced in the discharge space are drawn to the negatively charged cathode surfaces, including the insulated surface of the part 4I and by depositing on the latter surface, produce intense local gradients. At the boundaries of the insulating film patches 4I these gradients quickly become so great as to initiate cold cathode emission from the adjacent metal as exposed through the crevices a. This emission tends to increase rapidly in a cumulative fashion so that as soon as it is once initiated, an intense arc strikes almost immediately to the aiected region of the cathode part 28. The abundant electron emission thus realized completes the ionization of the discharge space and thus leads to complete breakdown of the tube.

The results obtained under the conditions speci; fied in the foregoing are extremely consistent and are reproducible with different tubes of similar design. This is a consequence of the fact that initiation of the discharge depends almost en` tirely upon emission' from the tungsten lament 21, and, since the characteristics of such laments "vary very little from sample to sample, one tube system which uses a boric acid solution as an elecaluminum in the present connection, it appears that its surface is of substantially the character indicated in Fig. 4 which shows on a magi-lined scale the end-portion ofthe aluminum tab 4I. This tab has on'i surface 'a thin insulating 111111 may be expected to perform in substantially the same manner as another.

It should be explained that the reason which requires the use of the auxiliary cathode part 28 in addition to the filament 2l lies in the inability of an uncoated Wire of small diameter (as is required in the instant case because of the limited heating power which can be provided in a projectile detonating unit or the like) to supply the relatively intense electron iiow required for a complete breakdown of the discharge path between the cathode structure and the anode. It is the function of the cathode part 28 to suppls7 the additional emission which is needed in this connection. However, in order that the cathode part'28 shall not function as an electron source during the initial starting of the discharge (and thus detract from the extremely consistent firing which the uncoated iilament assures) it is preferably sum'ciently separated from the filament (i. e to the extent shown) so that it is not, maintained thermionically emissive by heat received from the lament. 'In other words, the cathode part 28 functions exclusively as an auxiliary emitter which comes into operation only after pre-l liminary ionization' of the discharge space is produced by current flow 'from the filament. 2l.` 'Once' instant of breakdown may be said to be almost wholly governed by the emitting properties of the filament 21 and to be essentially independent of the properties of the auxiliary part 28.

It is not essential to the purposes of the invention that the insulating substance applied to the auxiliary cathode part be a lm of the type produced by anodizing, although this embodiment is preferred because it assures low voltage breakdown of the discharge path. Alternatively, one may employ other non-metallic insulating substances such asv nely divided aluminum oxide `(alumina), silica, or `even a material such as barium carbonate. (In connection with the use ofthe latter material, it will be understood that this materia1 is used mainly for the insulating qualities which it possesses when in an unheated state, rather than for its emission-enhancing properties.) Irrespective of the nature of the particular insulating substance involved, it is considered helpful, although not essential, to scratch or otherwise break the surface of the insulating layer so that at least some exposed metal surfaces are provided in direct contact with the insulating material.

VIt should also be pointed out that the auxiliary cathode part may be diierently located than in the arrangement shown in Fig. l, although the arrangement of the latter figure is advantageous frorna` constructional viewpoint. In some cases itwill serve merely to secure the treated metal tab to the inner surface of the header I8, and it is even possible to form the auxiliary cathode merely by applying a patch of an insulating substance to the header itself.

Tubes of the character described herein are particularly adapted for controlling the detonating circuit of a projectile which is desired to be fired by electrical means. In this application, which is that suggested by the arrangement represented in Fig. 1, it is apparent that the tube as a whole must` be capable of sustaining very great mechanical stresses such as are necessarily encountered during the firing of a projectile from a gun or cannon. For example, it may be readily calculated that the force due to the initial accelei'ation of an anti-aircraft shell is from 20,000 to 30,000 times that due to gravity. The envelope construction which has been described herein is found admirably adapted to sustain forces of this character without fracture. This is in part a consequence of the fact that the forces exerted on the various seals of the tube and on the tubes glass wall parts are mainly compressive in character, this being a favorable condition from the standpoint of yielding maximum strength. It has further been found advantageous to give the various electrode parts a bowed configuration and in this connection the anode header I1 is shown as being bowed in the upwar'd direction so that it can adequately resist the forces exerted on it by the tubulation 20 during initial acceleration of the projectile I I. The grid 23 is given a downward flexion by a prelrninary stretching of its fabric so that no appreciable change in the cathode to grid spacing occurs when the shell is iired from the gun.

The cathode header I8 is adequately reinforced by being seated against a transverse mounting block 45 which is recessed to receive the main structure of the header while providing appropriate backing for the glass-to-metal seal between the header and theglass bart I4'. 'A lead washer 46 is interposed between the outwardly extending peripheral portion of the header I8 and the surface of the mounting block 45 so as to provide a deformable seat for the tube. A slight spacing (e. g. a few mils) is left between the bottom of the header and the floor of the recess with- Fin which the header projects so as to allow a slight motion of the tube during high acceleration periods and to assure proper seating of the tube on the gasket 46. A terminal connection for the header I8 is provided by securing to it a conductor 45.

The mode of use of a tube of the character specified in a projectile detonating system is in` dicated in Fig. 5 of the drawing. In the illus'lv trated circuit the electrodes I1', 23', 21' and 28" are assumed to correspond to the similarly numbered parts shown in Fig. 1. The cathode filament-21' has associated with it a battery 5|- (e. g; a small dry cell) which is adapted to be connected in series with the filament when the shell is red from a gun. To this end there is provided in the iilament circuit a switch 52 which may appropriately be of the centrifugal type S0 as to be closed automatically as soon as the shell is iired. The circuit between anode I'I' and cathode 28' 'includes a condenser 54 which is charged to a definite voltage before the shell is introducedinto the gun. This condenser is in series with a detonating element 55, for example, a fuse link, which is adapted to explode the charge in the shell Whenever `the condenser 54 discharges through it.

For controlling and timing the operation of the detonator, there is further provided in connection with the grid 23 a circuit which includes a condenser 51 in parallel with a resistor 58 and a xed-voltage battery 59. The condenser 51 is provided with a terminal 60 which permits it 'to be charged at the time the shell leaves the gun, the amount of charge being regulated by a firing predictor which controls a voltage source to which the condenser may be instantaneously connected. (The predictor and the voltage source are not illustrated.) Obviously, the condenser 51 has to be charged in a very short time (approximately ten microseconds) on account of the high muzzle velocity of the shell, so that the condenser should be as small as possible and the charging circuit sti but non-oscillatory. The polarity of the battery 59 is opposed to that of the condenser "51 when charged, and the resistor 53 is of relatively high value (e. g. 109 ohms) so as to give a relatively long time constant to the condenser circuit.

The operation of the system thus represented is illustrated graphically in Fig. 6. This figure shows that the grid potential,which is represented by the irregular line A, A', A", is maintained at a voltage V1 corresponding to the voltage of the battery 59, until the instant when the shell leaves the gun. The voltage V1 is favorable to a discharge between the cathode 21 and the anode I1' of the discharge tube, but such a discharge is prevented from taking place by the fact that the cathode 21 is at this time in an unheated condition. At the point a: at which the shell leaves the gun, the filament circuit is closed (i. e. by operation of the centrifugal switch 52), and the condenser 51 is charged to a, voltage Vu which renders it negative With respect to the cathode. Thereafter, the grid voltage immediately starts to rise again along the exponential curve A", and within a period determined by the time constant of the condenser circuit (as xed by the resistor 58) rises at y to a value which is equal to the critical firing voltage of the tube. At this point' current starts to flow from the cathode lament 2l', and the resulting ionization of the discharge space leads to the immediate establishment of `an arc discharge to the cathode part 28'. Since this involves a complete breakdown of the tube, the condenser 54 is abruptly discharged through the detonatorv 55, and the shell charge is exploded.

With the use of a tube construction of the type illustrated in Fig. 1, and assuming a xed anode voltage, it is found that various tubes may be fired by the use of .a grid voltage which varies with dilerent tubes by not 'more than about 0.1 volt. With a circuit such as that shown in Fig. `5, this signifies a time variation on the order of J/ioo of asecond, which is considered entirely satisfactory for the application in question. The described device has the further advantage for this application that it uses a very small cathode heating current and thus requires a correspondingly small battery for supplying this current. This is obviously an important consideration in view of the limited space available in a shell casing of usual dimensions.

While the invention has been described by reference to a particular embodiment thereof, it will be understood that numerous modifications may be made by those skilled in the art without departing from the invention. We, therefore, aim in the appended claims to cover all such equivalent variations as come within the true spirit and scope of the foregoing disclosure.

What we claim as new and desire to obtain by Letters Patent of the United States is:

1. A gas-filled discharge device including an anode and a cathode, said cathode including an exposed heatable lament of uncoated metal, a body of metal electrically connected to the filament but sufficiently separated therefrom to prevent thermionic emission from the body solely as a result of heat received from the filament, and an insulating substance on the surface of said body providing exposed boundaries therebetween for facilitating the initiation of eld emission from said body in the presence of ionization produced in the device by electron now from said filament.

2. A gas-filled discharge device including an anode and a cathode, said cathode comprising a heatable filament of uncoated tungsten, a body of aluminum electrically connected to the filament but suniciently separated therefrom to prevent thermionic emission from the body solely as a result of heat received from said filament, and an insulating lm on the surface of said aluminum body of the type characteristically produced by anodizing.

3. A discharge device including a gas-filled container having a metal header which closes one end of the container, an anode having a surface within the container, and a cathode, said cathode comprising an uncoated metal filament which has one terminal electrically connected to the said header and the other terminal insulated therefrom to permit heating of the lament by passage of current therethrough and a further part which is also electrically connected to the said header and which includes an exposed metal surface coated with an insulating substance adapted to facilitate initiation of held-emission from the said surface upon the occurrence of ionization of the gaseous filling of the container.

4. A discharge device including a gas-lled container having a metal header which closes one end of the container, an anode having a surface within the container, and a cathode, said cathode comprising an uncoated metal filament, a conductive member supporting one extremity of the filament from the said header, a conductor insulatingly sealed through the header and connected to the other end of the lament, a metal tab aflixed to the said supporting member in proximity to the extremity of the said filament, and an insulating substance on the surface of the said tab to facilitate initiation of field emission from the tab upon the occurrence of an ionizing discharge from the said filament.

KENNETH H. KINGDON. ELLIO'I'I J, LAWTON.

REFEREN CES CITED The following references are of record in the le of this patent:

UNITED STATES PATENTS Number Name Date v1,267,827 Whitney May 28, 1918 1,787,300 Alexanderson L Dec. 30, 1930 1,959,419 Freedman May 22, 1934 1,989,132 Edwards et al. Jan. 29, 1935 1,953,906 Edwards et al. Apn 13, 1934 

